Cannot place the forearm on the correct low position while I'm playing with the armature
Select your one of your arm-bones while in pose mode, and press R. You can then rotate that bone using the mouse. The bone rotates according to your screen, so go to front/back(numpad 1/ctrl + numpad 1) view to get straight rotation.

To go from the images you posted to what I posted, you need to rotate the upper arm and the lower arm =)

Image I posted: